Faculty of Kinesiology, Sport, and Recreation, Neuroscience and Mental Health Institute, Sensorimotor Rehabilitation Neuroscience Group, University of Alberta, Edmonton, AB T6G 2H9, Canada.
Appl Physiol Nutr Metab. 2021 Apr;46(4):346-355. doi: 10.1139/apnm-2020-0366. Epub 2020 Sep 30.
The present study was designed to () determine the time course of changes in motor axon excitability during and after neuromuscular electrical stimulation (NMES); and () characterize the relationship between contraction fatigability, NMES frequency, and changes at the axon, neuromuscular junction, and muscle. Eight neurologically intact participants attended 3 sessions. NMES was delivered over the common peroneal nerve at 20, 40, or 60 Hz for 8 min (0.3 s "on", 0.7 s "off"). Threshold tracking was used to measure changes in axonal excitability. Supramaximal stimuli were used to assess neuromuscular transmission and force-generating capacity of the tibialis anterior muscle. Torque decreased by 49% and 62% during 8 min of 40 and 60 Hz NMES, respectively. Maximal twitch torque decreased only during 60 Hz NMES. Motor axon excitability decreased by 14%, 27%, and 35% during 20, 40, and 60 Hz NMES, respectively. Excitability recovered to baseline immediately (20 Hz) and at 2 min (40 Hz) and 4 min (60 Hz) following NMES. Overall, decreases in axonal excitability best predicted how torque declined over 8 min of NMES. During NMES, motor axons become less excitable and motor units "drop out" of the contraction, contributing substantially to contraction fatigability and its dependence on NMES frequency. The excitability of motor axons decreased during NMES in a frequency-dependent manner. As excitability decreased, axons failed to reach threshold and motor units dropped out of the contraction. Overall, decreased excitability best predicted how torque declined and thus is a key contributor to fatigability during NMES.
()确定在神经肌肉电刺激（NMES）期间和之后运动轴突兴奋性的变化过程；() 描述收缩疲劳、NMES 频率与轴突、神经肌肉接头和肌肉变化之间的关系。8 名神经完整的参与者参加了 3 次会议。NMES 在 20、40 或 60 Hz 下通过腓总神经传递 8 分钟（0.3 s“开”，0.7 s“关”）。阈值跟踪用于测量轴突兴奋性的变化。使用超刺激来评估胫骨前肌的神经肌肉传递和产生力的能力。在 40 和 60 Hz NMES 的 8 分钟内，扭矩分别下降了 49%和 62%。只有在 60 Hz NMES 期间，最大抽搐扭矩才下降。在 20、40 和 60 Hz NMES 期间，运动轴突兴奋性分别下降了 14%、27%和 35%。NMES 后，兴奋性立即（20 Hz）和 2 分钟（40 Hz）和 4 分钟（60 Hz）恢复到基线。总的来说，轴突兴奋性的降低可以很好地预测 NMES 8 分钟内扭矩的下降情况。在 NMES 期间，运动轴突变得兴奋性降低，运动单位“脱离”收缩，这对收缩疲劳及其对 NMES 频率的依赖性有很大贡献。运动轴突的兴奋性以频率依赖性的方式在 NMES 期间降低。随着兴奋性的降低，轴突无法达到阈值，运动单位脱离收缩。总的来说，兴奋性的降低可以很好地预测扭矩的下降情况，因此是 NMES 期间疲劳的关键因素。
